Lead-acid secondary batteries have been employed for some time for a variety of applications requiring general purpose electrochemical storage. The advantages of lead-acid batteries include: low cost of manufacture, simplicity of design, reliability and relative safety when compared to other electrochemical systems. Relatively good specific power has enabled the widespread use of lead-acid batteries for starting, lighting and ignition (SLI) purposes for vehicular, e.g., automotive, marine and aviation, applications. The lead-acid system has also found widespread use as traction batteries such as, for example, in golf carts and boats. However, the widespread use of lead-acid batteries for electric cars as an alternative to fossil fuel transportation has been limited by the need for better specific energy and deep discharge cycle lifetime. Bipolar lead-acid batteries have shown increasing promise at overcoming these limitations.
Conventional lead-acid batteries usually consist of individual positive and negative lead grids pasted respectively with positive active material (PAM) and negative active material (NAM). The lead grids must be thick enough so that they are structurally strong enough to support the active material, for example, lead-based paste. The extensive use of lead for the lead grids also limits the specific energy of the battery. In addition, continuous deep discharge cycling causes a corrosion layer to form on the positive lead grid resulting in poor contact of the active material with the lead grids which, in turn, leads to loss of power and to lower deep discharge cycle life.
In a typical bipolar lead-acid battery design, each electrode includes an electrically conductive and electrolyte impervious sheet or plate which serves as a partition between the battery cells. The PAM is adhered to the positive side and NAM is adhered to the opposite, negative side. The bipolar electrodes are stacked parallel to and on top of each other with the positive side of each electrode facing the negative side of the adjacent electrode. The current is collected perpendicular to the plane of the thin plates at endplates terminating at both ends of the stack of bipolar plates. This arrangement allows for the possibility of batteries with lower internal resistance and, thus, higher specific power. With the bipolar battery design it is possible to choose lightweight conductive materials to construct bipolar electrodes that do not corrode under continuous deep discharge cycling. Despite the apparent advantages of bipolar lead-acid batteries, the substantial effort to develop these batteries has yet to yield a commercially viable product.
Several materials have been used for various bipolar plate designs to meet the critical demands placed on it within the lead-acid battery environment. A practical bipolar plate should offer: the structural integrity to support the active material yet be lightweight; resistance to the various corrosion mechanisms occurring on both the positive and negative sides of the bipolar electrode during cycling; and the ability to be inexpensively manufactured. Corrosion may render the surface of the bipolar plate non-conductive or it may result in the bipolar plate being perforated thus causing an electrical short between two adjacent cells and battery failure.
A bipolar battery plate consisting of a plastic or polymer-based composite made of electrically conductive fibers or particles embedded in a polymer matrix is disclosed by Fitzgerald et al U.S. Pat. No. 4,708,918. The fibers are composed of acid resistant glass coated with fluorine doped tin dioxide which are resistant to corrosive potentials on the positive side. This patent discloses a thin film or foil of lead adhered to the polymer-based (resin) layer by a graphite-filled epoxy polymer adhesive. Although the polymer-based composite reduces the need for corrosion protection, it also has somewhat limited conductivity properties.
Rippel et al in U.S. Pat. No. 4,275,130 discloses a polymer-based composite bipolar battery plate using carbon fibers or particles as the filler for electrical conductivity. The carbon fibers, while stable to the potentials on the negative side, quickly oxidize at the potentials on the positive side causing perforation of the plate, resulting in battery failure.
Bipolar battery plates based on metallic sheets inherently offer the possibility of better electrical conductivity and strength than their polymer-based composite counterparts. The earliest bipolar plates were constructed of a lead sheet which were unsuitable because of lead corrosion on the positive side and excessive weight.
Rao et al in U.S. Pat. No. 4,892,797 discloses a bipolar plate structure in which an intermediate double sided electrically conductive pressure sensitive adhesive film is laminated between electropositive and electronegative layers, both of which may be metal sheets. Rao et al does not suggest that this intermediate layer has corrosion protective properties.
Rao et al in U.S. Pat. No. 5,348,817 discloses a bipolar battery plate having a multilayered metallic substrate structured as C/A/B/D where the surfaces of C and D are the positive and negative sides, respectively. Layer C consists of a layer of lead or lead alloy, and/or conductive oxides of tin, titanium or ruthenium. Layer A consists of a layer of titanium or tin. Layer B consists of a layer of copper or tin. Layer D consists of a layer of lead or lead alloy or tin.
A particular embodiment of a bipolar battery plate, disclosed by Rowlette U.S. Pat. No. 5,334,464, includes a core sheet of titanium protected on the positive side from corrosion by a protective layer of fluorine doped tin dioxide and protected on the negative side by a layer of lead or carbon.
In both Rao et al U.S. Pat. No. 5,348,817 and Rowlette U.S. Pat. No. 5,334,464, a core metallic layer is disclosed which is protected from corrosive potentials by additional layers that are resistant to corrosion. If titanium is used as the central core substrate, additional protective layer(s) must be added to the positive and negative sides that are resistant to corrosive potentials on the positive or negative sides. These protective layers must be thick enough to be free of pinholes and defects which may expose the titanium layer to corrosive potentials. In the case where the protective layers are metals, thicker layers result in substantial weight being disadvantageously added to the bipolar plate.
The active material often undergoes change in volume, for example, of about 20%, during a typical charge/discharge cycle. If the active material becomes separated and loses electrical contact with the bipolar electrode, loss of power results. In the conventional monopolar lead-acid battery design, the active material for each electrode is normally supported within a lead grid structure. In the bipolar electrode design the active material is often adhered to a flat planar surface. The active material adheres much better to a grid-like structure, such as the lead grid in the monopolar electrode design, rather than to a planar structure as in the bipolar electrode design. Various methods have been suggested in the prior art to assist in the adhesion of the active material to the planar bipolar electrode. These methods include attaching a grid to the surface of the bipolar electrode such that the grid is essentially in continuous contact with the surface of bipolar electrode. However, in doing so, the interconnecting crystalline network in the active material is disadvantageously interrupted and weakened by breaking it up into smaller segments.
Although many advances have been made in lead-acid bipolar plate design, the commercial availability of a practical bipolar lead-acid battery has yet to be realized. The development of a bipolar plate to be used for the successful commercialization of bipolar lead-acid batteries suitable for the long sought desire of electric automobiles would be a major technological advance for both the transportation and electric power storage industries.